The use of diffusive gas sensors to detect the concentration level of gaseous species of interest using the photoacoustic effect is well known. For example, U.S. Pat. No. 4,740,086 teaches the use of a diffusive photoacoustic gas sensor to convert the optical energy of an amplitude modulated light source into acoustic energy when the light mechanically and thermically excites the gaseous species of interest as it diffuses into a sensing chamber upon which the light is incident. Sound waves of an intensity corresponding to the concentration level of the gas within the chamber are generated as the light radiation absorbed by the gas creates pressure fluctuations of a magnitude proportional to the number of gas molecules located within the sensing chamber. These sound/pressure waves are detected by an acoustic detector such as a microphone.
However, the output signal of a diffusive photoacoustic sensor is susceptible to noise created by interference from outside sources of air pressure fluctuations, such as wind, vibration and acoustic phenomena. To eliminate such noise, one may incorporate some means of attenuating extraneously generated pressure waves, while attempting to allow the gas to freely diffuse into the sensing chamber for detection. For example, porous members through which gas relatively readily diffuses, but which attenuate the effect of external pressure fluctuations, are often placed at the entrance of photoacoustic sensors. However, one must balance this attenuating effect with an increase in response time. In that regard, introduction of a sound/pressure attenuating element(s) to reduce noise typically results in a corresponding loss of responsiveness to changing signal levels. The specifications for combustible gas detectors of the Instrument Society of America (ISA) require gas concentration level measurement stability at wind speeds of up to 5 meters per second (m/s) with a corresponding response time (to 60% of full scale indication) of less than 12 seconds.
It is very desirable to develop devices and methods that increase signal-to-noise ratios in photoacoustic detectors and other gas sensors while maintaining a satisfactory response time for such detectors.